In recent years, methods and apparatus for creating evoked potentials in the neural pathways of higher level organisms (e.g., animals and humans) have been developed. Evoked potentials can produce observable movements and/or analyzable electric signals (e.g., brain waves). Evoked potentials are created by stimulating neural pathways. In the past, three major types of stimulators have been used to create evoked potentials--light, sound and electric stimulators. Light and sound stimulators have been used to stimulate the sensory neural pathways associated with the eyes and ears. Electric stimulators have been used to stimulate motor neural pathways and the sensory neural pathways associated with somatic sensations, i.e., sensations associated with the sense of touch. The present invention is directed to electric stimulation.
In the past, the most common way to electrically stimulate motor and somato sensory neural pathways has been to attach a pair of spaced-apart electrodes to the body at the stimulus location. When electric potential is applied to the electrodes a current flow through the body is created. The current flow produces an electric field that disrupts the polarization of neurons located in the field (commonly called depolarization of the neurons) causing an evoked potential "message" to be transmitted along the neural pathway formed by the depolarized and other neurons that define the neural pathway. When peripheral nerves are being stimulated in this manner, it is usual to attempt supramaximal stimulations so that all neurons are depolarized or "fired" simultaneously. This is done to eliminate variations from stimulus to stimulus and is readily accomplished by applying a current of adequately high magnitude for a sufficient period of time--typically 20-30 milliamperes for 100 microseconds in the case of the human body. In general, the pulse width of a stimulus (50-200 microseconds) is less than the decay time of the neurons to be stimulated (300 microseconds in the case of peripheral neurons). After the stimulus is removed, there may or may not be a reverse recovery current. Even when a recovery current occurs, it is limited to a value that will not itself cause depolarization.
While electric stimulation using a pair of spaced-apart electrodes has certain advantages, it has several disadvantages. One major disadvantage of electrode stimulators relates to the fact that the body is an insulator. As a result, the electric current flow between the electrodes is shallow, i.e., it occurs near the skin. Because current flow is shallow, the electric field created by the current flow is shallow. As a result, deep neurons are not depolarized and, thus, not stimulated. While current can be increased to increase current penetration depth and, thus, stimulation depth, high current flows cause pain and, thus, are undesirable. In fact, pain is one of the major reasons why the use of electrode stimulators to stimulate the brain can only be used on comatose patients. Awake patients generally cannot stand the pain associated with the high current flow needed to stimulate neurons enclosed by cranial bone.
A further disadvantage of electrode stimulators relates to a lack of stimulation selectivity. As noted above, when peripheral neural pathways are stimulated it is usual to attempt supramaximal stimulation. Supramaximal stimulation is used because it stimulates all motor and sensory neural pathways lying in the depolarizing electric field regardless of whether the neural pathways are formed of slow or fast neural fibers. Other stimulation techniques that utilize a spatial relationship are employed to test different speed neural pathways. For example, a collision technique that utilizes two electrode stimulators, one distally (e.g., at the wrist) and the other proximally (e.g., at the elbow), is used to study the slow neural fibers located in the forearm of a patient.
In order to overcome the shallow penetration disadvantage of electrode stimulators, proposals have been made to use a magnetic coil to create a neuron depolarizing electric field. Such devices, commonly called magnetic stimulators, have the advantage of being relatively pain free and noncontacting, as well as capable of stimulating deep and otherwise inaccesible nerves. Depth is improved because, unlike current flow, body tissue does not resist magnetic flux. While magnetic stimulators have the ability to provide deeper penetration with less pain, as best understood, neuron depolarization is still due to the creation of an electric field. More specifically, as best understood, the changing magnetic field created by a magnetic stimulator induces eddy currents in body tissue that, in turn, create a neuron depolarizing electric field.
While magnetic stimulators have certain advantages over electrode stimulators, prior magnetic stimulators have had disadvantages. One major disadvantage of prior methods and apparatus for magnetically stimulating neural pathways is their inefficiency. More specifically, in order to create the large magnetic fields needed to produce a depolarizing electric field in the body of a patient, a large current flow must be created in the coil of the magnetic stimulator. Large current flows are created by rapidly discharging a capacitor bank that has been charged to a high voltage level. Resistance in either or both the capacitor bank charging circuit and the coil discharge circuit produce a loss of energy. Lost energy reduces efficiency. Similarly, creating a capacitor bank charge greater than that needed to produce a depolarizing field results in less than a maximum efficiency system, as does discharging the capacitor bank charge by more than the amount needed to create neuron depolarization. Not only do these techniques create inefficiency, they also unduly prolong charge recovery time and, thus, extend the time between stimulations.
This invention is directed to a method of magnetic stimulation that overcomes the foregoing and other disadvantages and an apparatus for carrying out the method.